U0172

Universal

Lost Communication With Restraints System Sensor C

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Advanced
High Severity

The restraint system monitors communication with Sensor C so its control system can use information from that sensor when managing occupant protection. This code means communication with that sensor has been lost. The exact role of Sensor C depends on the vehicle. The SRS warning light may come on, and some restraint functions may not work as intended.

Report an error with this code

⚠️ Before You Begin (Safety First)

Treat this as a safety-related fault: airbags or seat-belt pretensioners may not provide their intended protection. Arrange diagnosis promptly and follow the vehicle’s owner’s manual. Do not probe, disconnect, or attempt to repair airbag or pretensioner wiring yourself. If the vehicle has been in a collision or the SRS warning light is on, have the restraint system checked by a qualified technician.

Common Symptoms

  • The SRS or airbag warning light may illuminate because the control system has detected lost communication.
  • Airbag or seat-belt pretensioner functions associated with the affected system may be limited or unavailable; the exact effect depends on the vehicle.
  • There may be no obvious change in normal driving because a communication fault can leave the vehicle’s usual operation unaffected.
  • Other restraint-system warnings or related communication codes may appear if the fault affects additional modules or network connections.

Common Causes

  • A loose, damaged, corroded, or poorly connected connector or wire may interrupt communication with Sensor C.
  • A fault in the sensor’s power or ground supply may prevent it from communicating.
  • A communication-network wiring fault or another network disruption may interrupt messages to or from the sensor.
  • Sensor C may have an internal fault, but the code alone does not establish that the sensor needs replacement.
  • A fault in a restraint-system control module or a related connection may prevent communication.

Diagnostic Steps

  1. Check the owner’s manual for the meaning of any SRS warning light and arrange prompt professional diagnosis if one is on.
  2. Note when the warning appeared and whether other electrical or warning-light issues occurred at the same time; this context can help diagnosis.
  3. With the vehicle safely parked, visually check for obvious damage to accessible wiring or connectors without touching, disconnecting, or probing SRS components.
  4. If you have a suitable scan tool, record the code and any related codes before clearing anything; do not clear the code as a substitute for diagnosis.
  5. Have a qualified technician identify Sensor C and test its circuits and network connections using vehicle-specific service information and SRS safety procedures.

When to Call a Professional

Use a scan tool that can communicate with the restraint system to confirm the fault and check for related codes across other modules. Identify Sensor C using vehicle-specific service information, then assess its power, ground, connectors, and communication wiring using approved SRS precautions and procedures. Do not replace the sensor based on this code alone.

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Commonly Asked Questions

Does U0172 prove that Sensor C has failed?

No. It reports lost communication, which can result from a sensor fault, a power or ground problem, damaged wiring, a connector issue, or a broader network or module fault.

Can I tell what Sensor C does from this code alone?

No. The code identifies the communication loss but does not specify the sensor’s exact function or location. Those details depend on the vehicle’s model, year, and engine.

Is it safe to keep driving?

This code does not by itself establish that the vehicle is unsafe to control, but restraint protection may be impaired. Follow the owner’s manual and arrange prompt SRS diagnosis; do not rely on airbags or pretensioners working as intended until the fault is resolved.

Should I replace Sensor C or clear the code?

Neither is a reliable first step. A technician should confirm the communication fault and test the relevant supply, ground, connectors, wiring, and network before recommending a repair.

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